Natural immunity comes in a lot of variations, and sometimes its even so weak that reinfection can occur. Immunity by vaccine is comparatively very predictable

Natural immunity comes in a lot of variations, and sometimes its even so weak that reinfection can occur. Immunity by vaccine is comparatively very predictable and by nearly all accounts at least as strong as most natural immunity, with plenty of studies finding it to be stronger. So, that is part of the concern. Additionally, antibody tests are not fully reliable, whereas being vaccinated is never going to be "wrong". It's simply you are or you aren't, with no chance of false positives.